Detection of hidden nodes of a station by the whole network 35

As far, no works have been done to achieve that kind of distributed detection for the hidden node problem. By this kind of detection, we mean that the whole network under-stands by itself an hidden node relation. For instance, in Shi-huas thesis, a station detect only its own hidden nodes, but not the ones of other stations. To achieve this detection, we use the timing properties of IEEE 802.11af networks. We define two periods as illustrated in Fig. 16, where station B is hidden to station A while station C is not hidden to station A.

Figure 14: B no hidden

The first period is called hidden period (HP). When a station attempts a transmis-sion, it first sends a RTS frame and then waits a SIFS time for the CTS response. If a station iattempt a transmission and is hidden to station j therefore station j will sense the channel idle until the reception of the CTS frame for station i. The HP represent this whole period, equal to RTS+SIFS, the channel is idle for station j instead of being busy because of the hidden node relationship between i and j. In Fig. 16, In the IEEE 802.11af standard, a RTS transmission time with the lowest data rate is 408us and a SIFS time is 120us. As a consequence, an HP is 408us+120us=528us=22 backoff slots, where the backoff slot length is 24 us. The second period is called collision period (CP). If a station attempts a RTS frame transmission while one of its hidden station is already transmitting a RTS frame, a collision of RTS frame happens. If there is no capture effect, then the AP cannot decode any frames and the both RTS transmissions failed. We define CP as the possible collision period between two hidden stations, as illustrated in figure. Therefore, a CP is equal to and RTS transmission time and in the IEEE 802.11 standard, CP is 408us=17 backoff slots.

Between the end of these two periods, there is an interesting gap of 5 backoff slots.

Indeed, if station A first transmits its RTS and station B starts transmitting its RTS in the

doi:10.6342/NTU201701832 4.2. DETECTION MECHANISM OF THE HIDDEN NODE RELATIONSHIPS IN IEEE

802.11AF NETWORKS. 37

Figure 15: B hidden

NCP of A , the transmission of A is still successful. The transmission of B does not have any effect on As. A receives a CTS and transmits its data and waits for an ACK from the AP. The AP sends an ACK to A and if B can hear this ACK, then B can understand that is it hidden to A. We are going to use this event as the key idea of our hidden node detection.

We take a simple network, where A and B are just hidden to each other but C is not hidden to A and B. A is the first station to transmit. During the transmission of its RTS, there is no collision so the AP can perfectly decode it and starts switching to the receiver mode during the first slot after the RTS reception. But station B, which is hidden to A, starts sending its RTS during the SIFS waiting time of station A. Then the AP has already switched from receiver mode to transmitter mode and cannot hear this RTS from B. Station B ends transmitting its RTS and start waiting for a SIFS time. At the same time, the AP has sent a CTS to station A, announcing that A can send its data. Station B cannot decode this CTS because it did not have the time to pass in receiver mode and so will not receive an CTS. Then,we introduce a new timeout T1, equal to 2*SIFS+dataLength , to allow station B to wait for the ACK addressed to station A and then understand it has transmitted in the NCP period of this station.

Figure 16: Hidden periods

As a matter of fact, if station B sends a RTS but does not receive its CTS frame after SIFS + 1 time slot, then station B starts counting down a timeout. If during this timeout the station decodes an ACK frame addressed to a station A, then station B understands it is hidden to station A and has transmitted its RTS the NCP period of station A.

Now, station B is the only one to know this information and we want it to inform the whole network. If station B transmits again one time, the other stations will understand a capture effect relationship as we design for the capture effect detection mechanism. To make a difference, we force station B to transmit again two times : the first time with the RTS/CTS mechanism and the second time by sending its data directly. Then the duration time in its RTS will be unique and equal to D3=2*dataLength+ACK+DIFS+3*SIFS. By doing this, station B will inform the whole network of its hidden node relationship with A as follows:

• from As perspective : the first transmission of station A is successful and station A starts counting down the timeout T2. Then, it decodes a CTS addressed to station

doi:10.6342/NTU201701832 4.2. DETECTION MECHANISM OF THE HIDDEN NODE RELATIONSHIPS IN IEEE

802.11AF NETWORKS. 39

Figure 17: Both issue: detection of the hidden node relationship

B with the duration time set to D3 . So, station A concludes it is hidden to B and sets its NAV.

• from other stations perspective : they decode the CTS addressed to station A and sets their NAV to the CTS As duration time. But, during this NAV, they decode a new CTS addressed to station B with a unique duration time set to D3 . So, they understand station B is hidden to station A due to this specific duration time and they set their NAV to the CTS Bs duration time.

4.2.3 Special scenario : station A and B are hidden and station A cap-tures station B

In this scenario, we separate the capture effect and the hidden node informations as if they were independent. The whole network will be informed of these two relations in separated ways and at different moments, depending on the circumstances of the collision between A and B.

4.2.3.1 detection of the hidden node relationship

The collision between station A and B leads to the detection of their hidden node relationship if station A is the first station to transmit and station B transmits in the HP of station A, at least one slot after As transmission starts. As a matter of fact, in these

circumstances, B is too late to decode the CTS addressed to A. As shown in Fig. 18 , station B starts the timeout T2, receives the ACK addressed to A and then transmits two times.

This double transmission informs the whole network that A and B are hidden to each other.

Figure 18: Both issue: detection of the hidden node relationship

4.2.3.2 detection of the capture effect relationship

The collision between station A and B leads to the detection of their capture effect relationship if station B is the first station to transmit and station A transmit in the HP of station B, at least one slot after Bs transmission start, or if A and B transmit at the same time. As a matter of fact, in these circumstances, B can decode the CTS addressed to A.

As shown in Fig. 19 , station B starts the timeout T1 and receives the CTS addressed to A during the 17 first slots of this timeout. Then station B understands it has been captured by A and transmits one time to inform the whole network of this relation.